mbedtls: allow to use a platform specific config.h file

A platform specific mebedtls_config.h can now be defined if needed
by the parent makefile.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I532284a72d27424cc801874d7cd0caffe90f2011
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184838


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
Tested-by: Lionel DEBIEVE <lionel.debieve@st.com>

drivers/auth/mbedtls/mbedtls_common.mk

@@ -16,7 +16,7 @@ endif
 MBEDTLS_INC		=	-I${MBEDTLS_DIR}/include
 
 # Specify mbed TLS configuration file
-MBEDTLS_CONFIG_FILE	:=	"<drivers/auth/mbedtls/mbedtls_config.h>"
+MBEDTLS_CONFIG_FILE	?=	"<drivers/auth/mbedtls/mbedtls_config.h>"
 $(eval $(call add_define,MBEDTLS_CONFIG_FILE))
 
 MBEDTLS_SOURCES	+=		drivers/auth/mbedtls/mbedtls_common.c